NFC stops working after a few minutes?

I am also having some issues with NFC. I got 2 pucks. On one the NFC URL works just fine. The other stopped working after a few hours of fooling around with it. For example, if write something like:

NRF.nfcURL('https://www.google.com');

and upload to both pucks, it works fine on one and not on the other. That said, I can not get the following code to work on either of my pucks:

NRF.on('NFCon', function() {
  digitalPulse(LED1, 1, 250);
});

I was hoping to use NFC to be able to tap one puck on the other and pass some information. I have tried resetting the troublesome puck, but haven't had any luck. All other functions tested on the pucks have worked fine so far (bluetooth, mag, temp, etc.)